The following references are incorporated herein by reference in their entirety:    U.S. Pat. No. 8,404,302    U.S. Pat. No. 8,492,189    U.S. Pat. No. 9,070,803    US20120255602A1    CN203212630U    CN101429643A    CN101572279A    CN101582468A    CN101882632A    CN102044593A    CN102199759A    CN102623569A    JP2004190052A    BANERJEE, A. et al., Nanocrystalline ZnO Thin Film Deposition on Flexible Substrate by Low-temperature Sputtering Process for Plastic Displays, (Research Paper), Journal of Nanoscience and Nanotechnology, October 2014, Pp. 7970-7956, Vol. 14, No. 10.2011.    TOHSOPHON, T. et al., High Rate Direct Current Magnetron Sputtered and Texture etched Zinc Oxide Films for Silicon Thin Film Solar Cells, Proceedings for 6th International Conference on Coatings on Glass and Plastics (ICCG6)-Advanced Coatings for Large-Area or High-Volume Products, Jun. 14, 2007, Pp. 4628-47632, Vol. 516, No. 14.    MIORIN, E. et al., Textured Transparent Conductive Oxide Thin Films with Uniform Properties on Large Scale.    DEWAN, R. et al., Analyzing Periodic and Random Textured Silicon Thin Film Solar Cells by Rigorous Coupled Wave Analysis, Aug. 12, 2014.    ZHIFANG, L. et al., Development of Aluminum-doped ZnO Films for a-Si:H/_c-Si: H Solar Cell Applications, (Research Paper), Journal of Semiconductors, June 2013.
The following cited references are also incorporated herein by reference in their entirety:    [1]. J. Bailat, L. Fesquet, J.-B Orhan, Y. Djeridane, B. Wolf, et. al.; “Recent Developments of High-Efficiency Micromorph® Tandem Solar Cells in KAI-M PECVD Reactors”, Processing of the 25th European Photovoltaic Solar Energy Conference and Exhibition/5th World Conference on Photovoltaic Energy Conversion, 6-10 Sep. 2010, Valencia, Spain (2011) p. 2720-2723.    [2]. P. Obermeyer, D. Severin, M. Kress, S. Klein, U. I. Schmidt, S. Wieder “Textured ZnO enables high-efficiency silicon solar modules”, Renewable Energy, July (2010)    [3]. R. H. Horng, S. H. Huang, C. C. Yang and D. S. Wuu; “Efficiency Improvement of GaN-Based LEDs with ITO Texturing Window Layers Using Natural Lithography”, IEEE Journal of Quantum Electronics, Vol. 12, (2007) p. 1196-1201.    [4]. S. Li, D. S. Kuo, C. H. Liu, S. C. Hung, S. J. Chang; “Efficiency improvement of GaN-based light-emitting diodes by direct wet etching of indium-tin-oxide layer”, IET Optoelectron., 2012, Vol. 6, p. 303-306.    [5]. S. Nicolay, M. Benkhaira, L. Ding, J. Escarre, G. Bugnon, F. Meillaud and C. Ballif; “Control of CVD-deposited ZnO films properties through water DEZ ratio Decoupling of electrode” Solar Energy Materials & Solar Cells, Vol. 105 (2012) p. 46-52.    [6]. C David, T Girardeau, F Paumier, D Eyidi, B Lacroix, N Papathanasiou, B P Tinkham, P Guerin and M Marteau; “Microstructural and conductivity changes induced by annealing of ZnO:B thin films deposited by chemical vapour deposition”, Journal of Physics: Condensed Matter, Vol. 23 (2011).    [7]. M. Despeisse, C. Ballifa, A. Feltrina, F. Meillauda, S. Faya, F-J. Hauga, D. Dominéa, M. Pythona, T. Soderstroma, P. Buehlmanna, G. Bugnona, G. Parascandolo; “Research and Developments in Thin-Film Silicon Photovoltaics”, Proceeding of SPIE, Vol. 7409 (2012).    [8]. M. Boccard, P. Cuony, C. Battaglia, M. Despeisse, and C. Ballif; “Unlinking absorption and haze in thin film silicon solar cells front electrodes”, Phys. Status Solidi RRL, Vol. 4, (2010) p. 326-328.    [9]. J. Steinhauser, S. Meyer, M. Schwab, S. Faÿ, C. Ballif, U. Kroll, D. Borrello; “Humid environment stability of low pressure chemical vapor deposited boron doped zinc oxide used as transparent electrodes in thin film silicon solar cells”, Thin Solid Film, Vol. 520 (2011) p. 558-562.    [10]. D. W. Sheela, H. M. Yatesa, P. Evansa, U. Dagkaldiranc, A. Gordijnc, F. Fingerc, Z. Remesd, M. Vanecekd, “Atmospheric pressure chemical vapour deposition of F doped SnO2 for optimum performance solar cells” Thin Solid Films, Vo. 517, (2009) p. 3061-3065.    [11]. H. M. Yatesa, P. Evansa, D. W. Sheela, S. Nicolayc, L. Dingc, and C. Ballifc; “High-performance tandem silicon solar cells on F:SnO2”, Surface and Coatings Technology, Vol. 230, (2013) p. 228-233.    [12]. K. M. Chang, P. C. Ho a, A. Ariyarit, K. H. Yang, J. M. Hsu, C. J. Wu, C. C. Chang; “Enhancement of the light-scattering ability of Ga-doped ZnO thin films using SiOx nano-films prepared by atmospheric pressure plasma deposition system”, Thin Solid Films, Vol. 548 (2013), p. 460-464.    [13]. X. Yan, S. Venkataraj, A. G. Aberle; “Modified Surface Texturing of Aluminium-Doped Zinc Oxide (AZO) Transparent Conductive Oxides for Thin-Film Silicon Solar Cells”, Proceedings of PV Asia Pacific Conference 2012, Vol. 33 (2013), p. 157-165.    [14]. M. Vanecek, O. Babchenko, A. Purkrt, J. Holovsky, N. Neykova, A. Poruba, Z. Remes, J. Meier, and U. Kroll; “Nanostructured three-dimensional thin film silicon solar cells with very high efficiency potential”, Applied Physics Letters, Vol. 98 (2011).    [15]. M. M Hilali, S. Yang, M. Miller, F. Xu, S. Banerjee and S. V. Sreenivasan; “Corrigendum: Enhanced photocurrent in thin-film amorphous silicon solar cells via shape controlled three-dimensional nanostructures” Nanotechnology, Vol. 24 (2013)    [16]. A. Bessonov, Y. Cho, S. J. Jung, E. A. Park, E. S. Hwang, J. W. Lee, M. Shin, S. Lee; “Nanoimprint patterning for tunable light trapping in large-area silicon solar cells”, Solar Energy Materials & Solar Cells, Vol. 95 (2011) 2886-2892.    [17]. K. Y. Yang, K. M. Yoon, S. W. Lim, H. Lee; “Direct indium tin oxide patterning using thermal nanoimprint lithography for highly efficient optoelectronic devices”, Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, Vol. 27 (2009) 2786-2789.
Transparent semiconducting thin film coatings have been widely used in photovoltaic, display and solid-state lighting industries due to their good electrical conductivity and optical transparency. To enhance the device performance of solar cells and light-emitting diodes (LEDs), surface-texturing of transparent conducting thin film is generally required for improving light trapping or light extraction properties in the transparent semiconducting thin film deposition processes.
A well-known technique for forming surface-texturing on a transparent semiconducting film is by wet-etching method that uses additional acid or alkaline solutions to react with the film surface. The method is effective to create texturing surface. However, the surface may be over etched by the chemicals to cause some pin-hole defects. The residue of the chemical may also form some localized defects on the layer. In addition, the post-etching process is difficult to apply in a large-scale production since the control should be precise for retaining a good reproducibility.
Chemical vapor deposition (CVD) is another promising method for forming texturing transparent conducting thin film. In industry, CVD is widely used to produce good performance transparent texturing films with good reproducibility, even in a large-scale production. However, this method would require complicated chemical reactions under high temperature as well as expensive toxic gases, thus leading to additional costs and environmental issue.
For photovoltaic applications, research team from Neuchatel University and Oerlikon solar [1] investigating thin film silicon solar cell has reported that the textured TCO electrodes allows a 40% reduction of absorber thickness to achieve the same conversion efficiency in the Micromorph devices, which was reduced from 1300 nm to 780 nm, because of the improvement of the light trapping effect. Applied Materials [2] have shown that the surface roughened TCO increases the effective light path so as to enhance the long wavelength (650 to 1100 nm) absorption in a tandem solar cell, which generates approximately 15% more current (FIG. 3). For lighting and display applications, the surface-textured TCO window layers of the LEDs not only solve the current spreading problems but also reduce the problematic internal reflections, leading to a higher brightness LED. A study on GaN based LED has reported that extraction quantum efficiency of the ITO/GaN LEDs with and without textured surface is 22.6% and 17.4%, respectively [3]. A similar investigation using wet etch to form the texture on ITO layer has found that output power of the LEDs after etching is 21% higher than that of conventional LEDs [4]. Low Pressure Chemical Vapor Deposition (LPCVD) technology produces high quality textured boron doped ZnO (BZO) and this TCO material has been widely used in the silicon thin film photovoltaic industry and LCD display panel industry, which is adopted by Oerlikon and Tokyo Electronics. During the BZO deposition process, the deposition occurs as a result of chemical reactions of vapor phase precursors on a heated substrate. Diethylzinc (DEZ, (C2H5)2Zn), which structure is shown in FIG. 4, and water vapor (H2O) are used as precursors [5-6]. As the DEZ is a metal organic compound, the process is also called metalorganic chemical vapor deposition (MOCVD). The hydrolysis reaction that leads to the formation of ZnO from DEZ and water vapor during CVD is described via the equations:Zn(C2H5)2+H2O→C2H5ZnOH+C2H6 C2H5ZnOH+C2H5ZnOH→(C2H5ZnOH)2 C2H5ZnOH→ZnO+C2H
The chemical reactions are complicated and require relatively thick TCO material [7-8] which is about to be 2 to 5 um to form the textures (FIG. 5). The process uses some highly toxic processing gases, such as Diethyl zinc and Diborane, and expensive LPCVD vacuum systems. Therefore, the manufacturing process is high cost and induces environmental problems. Finally, a serious technical concern is that this BZO material will be degraded and lost its electrical conductivity when the materials are exposed to atmosphere moisture environment for several days [9] without further protection, thus limiting this technology to be in-house production use only. Atmospheric Pressure Chemical Vapor Deposition (APCVD) is another commonly used commercial technology to produce the textured fluorine doped tin-oxide (SnO2:F) coatings, which is currently adopted by Pilkington, Nippon Sheet Glass and Asahi Glass. In this APCVD process, the substrate is heated to 600 to 1,000 degree C. during the glass forming process on the tin bath, and the precursor gases namely tin tetrachloride, hydrogen fluoride and water vapor are reacted on the substrate [10-11]. The films can be grown with a textured surface morphology. The chemical reaction is described by the following formula:SnCl4+H2O+HF→SnO2:F+HCl
The optical absorption coefficient in the visible range is often quite high (˜400 to 600 cm-1) for the commercial FTO. Similar to the LPCVD technology, APCVD also involves not very environmentally friendly chemical process, expensive equipment and toxic gases (tin tetrachloride and hydrogen fluoride). Apart from the above concerns, high deposition temperature, lower optical transmission in UV & NIR regions and poor plasma resistance (FIG. 6) have limited the choices of substrate materials and induced lots of process constrains [12].
To get rid of expensive and complicated CVD processes, physical vapour deposition sputtering processing followed by wet chemical etching has been developed to form textured ZnO TCO coatings, which has been adopted by Ulvac and XinYi glass. In the sputtering process, atoms are physically removed from the solid target due to the bombardment by energetic particles such as argon ions. The ejected atoms are deposited on the substrate to form a thin film. A scheme of a sputter deposition system is depicted in FIG. 7. A negative high voltage can be applied to the target, a glow discharge of the gas is generated and the plasma is formed under the electric field between target and substrates. Compared to using the CVD processes, this processing requires a relatively lower processing temperature (not higher than 400 degree C.) and normally not involves using toxic gases. The equipment cost is less expensive. However, deposition of extra TCO material (Total material thickness of 4 to 5 m before etching) is required to form texture after wet etch, which is very difficult to control, leading to high material usage, low yield and pin-hole defects (FIG. 8) [13].
In some literatures, 3D patterning of TCO based of nanorods has attracted lots of attention for its potential light scattering effects on thin film solar cells. Vanecek et. al [14] presented results of optical modeling, where he predicted this TCO can greatly improve the efficiency of thin film silicon tandem cell from 11% to 15% with 60% reduction on absorbing materials. The design concept is aiming at decoupling the light path (a long path along the pillar) from the current path (perpendicular to the nano-rod). This permits the TCO to be thinner (˜100 nm perpendicular to the pillar) and lower low light induced degradation while still having large thicknesses for the absorption of the light. However, these nano-pillars are not easy to scale up for manufacturing. SEM images of hydrothermally grown ZnO pillars are shown in FIG. 9, but the distance between the pillars is difficult to control [15].
Recently, Samsung electronics and Korea University have developed the nano-imprint lithography fabrication to form a highly textured TCO and the process flow is shown in FIG. 10. A textured polymeric layer covered with pyramidal transparent conductive oxide structures is fabricated with a 5% improved transmission in UV-visible range [16-17]. However, nano-imprint technology is difficult to scale up and the process is very expensive.
A solution to minimize the index matching issues and strengthening the adhesion between the substrates and the transparent conductive coatings is therefore needed. In addition, a green and simple direct formation of textured TCO film and related process are highly desirable.
In U.S. Pat. No. 8,404,302, a solution process is employed which involves using metal oxide nanoparticles for forming the patterned layer as the first layer while the second layer will overlay on top of the first layer. As above mentioned, solution process is efficient to texture a film surface but it may cause defects and not suitable for large-scale production.
In U.S. Pat. No. 9,070,803, molecular imprint is employed which firstly forms a pattern layer and then deposits another conducting material on the patterned layer, which is a more complicated process. In addition, the disclosed method in this patent is more suitable for amorphous silicon PIN structure.
In CN102044593A, although sputtering is used in the deposition process, the disclosed method involves an additional formation of a patterned metal layer before forming the nano-texture TCOs thereon. It is not a direct texturing method on the TCOs which the present invention is intended to employ.